Frequency-domain optical coherence tomography with extended field-of-view and reduction of aliasing artifacts

ABSTRACT

The present disclosure provides an OCT imaging system to reduce or eliminate frequency-domain aliasing artifacts. The frequency is shifted using a carrier frequency to define a sampling range substantially centered on the carrier frequency. An image of the sample is generated from a displayed imaging range that consists of a subset of the frequencies within the sampling range. Furthermore, the system may be configured to determine the carrier frequency such that a Nyquist frequency corresponding to the shifted frequency is extended beyond either an upper or a lower bound of an OCT quality envelope corresponding to the first portion of light. Additionally, the carrier frequency may be determined such that a lower bound of the OCT quality envelope is greater or less than a zero-frequency DC limit.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. patent applicationSer. No. 14/278,424 filed on May 15, 2014, which claims priority to U.S.Provisional Patent Application Ser. No. 61/824,687 filed on May 17,2013, entitled “FREQUENCY-DOMAIN OPTICAL COHERENCE TOMOGRAPHY WITHEXTENDED FIELD-OF-VIEW AND REDUCTION OF ALIASING ARTIFACTS,” which bothapplications are incorporated herein by reference in their entirety.

TECHNICAL FIELD

In various embodiments, the present disclosure relates to opticalimaging systems, in particular optical imaging systems utilizingfrequency-domain interferometry.

BACKGROUND

Frequency-domain (or “swept-source”) optical coherence tomography (OCT)systems are powerful tools that provide non-invasive, high-resolutionimages of biological samples at higher acquisition speeds and lowersignal-to-noise ratios than time-domain OCT systems. FIG. 1 illustratesan exemplary frequency-domain OCT system 100 at a high level. As shown,the exemplary OCT system includes a wavelength-swept laser source 95(also referred to herein as a frequency swept source) that provides alaser output spectrum composed of single or multiple longitudinal modesto an input of a coupler 72. The coupler 72 divides the signal fedthereto into the reference arm 80 that terminates in the referencemirror 82 and the sample arm 84 that terminates in the sample 86. Theoptical signals reflect from the reference mirror 82 and the sample 86to provide, via the coupler 72, a spectrum of signals that are detectedby a photo-detector 88.

FIG. 2 is a plot 200 graphically illustrating a detection andranging-depth arrangement for a typical frequency-domain OCT system,such as, for example, the system 100 of FIG. 1. As depicted, an envelopeof coherence function 210 (or “fringe visibility curve”) defined by theinstantaneous output spectrum of the swept source and the detectionfrequency of the system is plotted in the frequency domain. In thisexample, both the source output spectrum and the fringe visibility curveare Gaussian. As will be appreciated, the positive and negativefrequency bands are not differentiable in the electrical domain.Accordingly, the images associated with the positive and negativefrequency bands, respectively, are overlapped. As a result of thisambiguity, only half of the frequency range, corresponding to positivedepth, is used for the imaging range 220. The upper frequency bound ofthe imaging range 220 is typically matched to the 6 dB roll-off (Zc) ofthe coherence function 210, which is referred to as the ranging depth.

The limitation on ranging depth (or the imaging depth range) illustratedin FIG. 2 has been ameliorated to some extent via the incorporation ofthe carrier-frequency heterodyne detection scheme described in U.S. Pat.No. 7,733,497 (the '497 patent), the entire disclosure of which isincorporated by reference herein. FIG. 3 is a plot 300 graphicallyillustrating a shifting of the frequency band by a carrier frequency(fs) 301 in accordance with the scheme of the '497 patent. As depicted,an envelope of coherence function 310 is shown. The function 310 isdefined by the instantaneous output spectrum of the swept source and thedetection frequency band, wherein the frequency band is shifted by thecarrier frequency 301. As will be appreciated, this shifting doubles theranging depth 320, as both sides of the frequency band centered at thecarrier frequency (fs) 301 produce images without ambiguity.Additionally, as shown, the Nyquist frequency (f_(nyquist)) 303 istypically double the carrier frequency (fs) 301. Furthermore, theNyquist frequency, as known in the art, may be one-half of the samplingrate of the system.

However, artifacts that result in sub-optimal imaging may in somecircumstances, plague even such carrier-frequency heterodyne detectionschemes. For example, FIG. 4, shows the plot 300 and further illustratesfoldover artifacts 331 (i.e., aliasing artifacts). The artifacts 331 mayresult from portions of the coherence function 310 beyond the DC (i.e.,frequency=0) limit and/or the Nyquist frequency limit. Furthermore, theartifacts 331 may manifest even when the frequency band has been shiftedby a particular carrier frequency (fs) 301. Such artifacts may be causedby reflections from structures outside of the imaging range of thesystem (due to, e.g., non-optimal sample placement) and may lead toaberrations in the OCT images. Thus, there is a need forfrequency-domain OCT systems and techniques that eliminate such aliasingartifacts while enabling larger imaging ranges.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a frequency-domain OCT system.

FIG. 2 illustrates a plot of a coherence function and associated imagingrange.

FIG. 3 illustrates a plot of a coherence function and associated imagingrange due to a shifting of the frequency band by a carrier frequency.

FIG. 4 illustrates a plot of the imaging range of FIG. 3 showingaliasing artifacts.

FIG. 5 illustrates a block diagram of a frequency-domain OCT systemarranged according to at least one embodiment of the present disclosure.

FIG. 6 illustrates a plot of a sampling range and correspondingdisplayed imaging range in accordance with the present disclosure.

FIG. 7 is a logic flow for optically imaging a sample utilizing afrequency-domain OCT system in accordance with the present disclosure.

FIG. 8 illustrates another plot of a sampling range and correspondingdisplayed imaging range in accordance with the present disclosure.

FIG. 9 illustrates an example OCT probe and corresponding frequencyscan.

FIG. 10 illustrates another plot of a sampling range and correspondingdisplayed imaging range in accordance with the present disclosure.

FIG. 11 illustrates another plot of a sampling range and correspondingdisplayed imaging range in accordance with the present disclosure.

FIGS. 12A-12B illustrate example images acquired using an OCT systemsimilar to the system described with respect to FIGS. 1-4.

FIGS. 12C-12D illustrate example images acquired using an OCT system inaccordance with the present disclosure.

DESCRIPTION OF EMBODIMENTS

In general, embodiments of the present disclosure may be implemented toreduce or eliminate aliasing artifacts in frequency-domain OCT systems.In particular, the present disclosure may be implemented to improve OCTimage quality.

With some examples, an image of a sample may be acquired using afrequency-domain OCT system wherein the frequency is shifted using acarrier frequency to define a sampling range substantially centered onthe carrier frequency. The system may generate an image of the sampleover a displayed imaging range that consists of a subset of thefrequencies within the sampling range. Said differently, a displayedimaging range may be generated wherein the displayed imaging rangecorresponds to a subset of the range of frequencies within the samplingrange. Furthermore, the system may be configured to determine thecarrier frequency such that a Nyquist frequency corresponding to theshifted frequency is extended beyond either an upper or a lower bound ofan OCT quality envelope. Additionally, the carrier frequency may bedetermined such that a lower bound of the OCT quality envelope isgreater or less than a zero-frequency DC limit.

Accordingly, an image of a sample can be acquired using afrequency-domain OCT system wherein foldover artifacts are not aliasedat least until they reach the outer bounds of the sampling imagingrange. The displayed imaging range is then generated from a subset ofthe frequencies in the sampling range to reduce and/or substantiallyeliminate foldover artifacts.

It is important to note, however, that the disclosed systems and methodsmay be embodied in many different forms and should not be construed aslimited to the embodiments set forth herein. Rather, these embodimentsare provided so that this disclosure will be thorough and complete, andwill fully convey the scope of the claims. In the drawings, like numbersrefer to like elements throughout.

FIG. 5 shows a high level diagram of a frequency-domain OCT system 500,arranged according to embodiments of the present disclosure. The system500 may be implemented to generate an image of a sample as describedabove. In particular, the system 500 may be implemented to select acarrier frequency for shifting the signal frequency band such that anOCT quality envelope corresponding to the shifted frequency band iswithin the bounds of the zero-frequency DC limit and the Nyquistfrequency limit. This will be explained in greater detail below.

The system 500 includes a wavelength-swept light source 95 that providesa light having an output spectrum composed of single or multiplelongitudinal modes. The source 95 provides the light to an input of acoupler 72. The coupler 72 divides the signal fed thereto into areference arm 80 and a sample arm 84. The reference arm 80 terminates inthe reference mirror 82, also referred to as a reference plane. Thesample arm terminates in a sample 136. Optical images reflected from thesample 136 and the reference mirror 82 are received by a photodetector88 and processed by a signal processor 160.

Additionally, the system 500 includes a single mode-fiber interferometeremploying an optical frequency shifter 311 in the reference arm 80. Insome examples, The frequency shifter 311 may include or consistessentially of, for example, an acousto-optic frequency shifter, asdescribed in the '497 patent. The optical frequency shifter 311 in thereference arm 80 shifts the signal frequency band by a carrier frequencyas described above. The signal processor 160 demodulates the carrierfrequency. With some examples, the frequency shifter 311 is in thesample arm (not shown). In some examples, multiple frequency shifters311 are provided (not shown) with a frequency shifter 311 disposed inboth the reference arm 80 and the sample arm 84.

In general, the signal processor 160 is configured to receive lightreflected from the reference plane 82 and the sample 136. Moreparticularly, light emitted from the source 95 is reflected from thesample 136 and the reference plane 82 and received by the signalprocessor 160. The signal processor 160 is configured to receive thisreflected light and generate the displayed image range from the samplerange. In particular, the signal processor 160 is configured to omitfrequencies from the set of frequencies corresponding to the samplerange to form the displayed image range. In some examples, the signalprocessor 160 may include a band-pass filter. The band-pass filter mayinclude both a high-pass filter and a low-pass filter. The high-passfilter may be implemented to reduce DC noise. Likewise, the low-passfilter may be utilized as an anti-aliasing filter. That is, the low-passfilter may be implemented to filter out frequencies higher than thebounds of the coherence function that might result in aliasingartifacts. As such, the low-pass filter may reduce aliasing artifacts.

The signal processor 160 may be realized as software, hardware, or somecombination thereof. The processor may also include a main memory unitfor storing programs and/or data relating to the methods describedherein. The memory may include random access memory (RAM), read onlymemory (ROM), and/or FLASH memory residing on commonly availablehardware such as one or more ASICs, FPGAs, electrically erasableprogrammable read-only memories (EEPROM), programmable read-onlymemories (PROM), programmable logic devices (PLD), or read-only memorydevices (ROM). In some embodiments, the programs may be provided usingexternal RAM and/or ROM such as optical disks, magnetic disks, or otherstorage devices.

For embodiments in which the functions of the processor are provided bysoftware, the program may be written in any one of a number ofhigh-level languages such as FORTRAN, PASCAL, JAVA, C, C++, C#, LISP,PERL, BASIC or any suitable programming language. Additionally, thesoftware can be implemented in an assembly language and/or machinelanguage directed to the microprocessor resident on a target device.

FIG. 6 is a plot 600 graphically illustrating a shifting of thefrequency band by a carrier frequency (fs) 601 in accordance with thepresent disclosure. In general, the carrier frequency (fs) 601 has beenselected to extend the Nyquist frequency (f_(nyquist)) 603 beyond anupper bound of an OCT quality envelope 640. As used herein, the OCTquality envelope corresponds to an intensity envelope of frequencies. Insome examples, the OCT quality envelope may be bounded by the 6 dBroll-off in measured intensity from the sample.

As will be appreciated, the OCT quality envelope 640 is related to andmay be dependent upon a variety of properties. In particular, the OCTquality envelope 640 may be related to: the coherence function of theswept source; properties of the optical beam illuminating the sample136, that is properties of the light emitted from the frequency-sweptlight source 95; the working distance, the waist, and/or the shape ofthe emitted light; and/or properties of the sample 136 beingilluminated, such as, for example, scattering and/or absorptionproperties of the sample 136.

As depicted in FIG. 6, the OCT quality envelope 640 is substantiallycentered about the displayed imaging range. Accordingly, the imagingdepth range can be decoupled into (a) a sampling imaging range 650 and(ii) a displayed imaging range 660. More specifically, the imaging rangecan be split into a sampling range 650, which corresponds to the depthrange of all sampled points. In particular, the sampling range 650 maycorrespond to substantially all of the depths defined by the frequenciesof the OCT quality envelope 640; while the displayed imaging range 660corresponds to the depth range displayed by the OCT system. That is, thedisplayed imaging range 660 corresponds to a subset of the set offrequencies of the sampling range 650.

With some examples, the present disclosure provides selection of thecarrier frequency (fs) 601 such that the sampling range 650 is definedbetween the DC limit 605 and the Nyquist limits 603. The imaging range660 is generated from the sampling range to reduce and/or substantiallyeliminate foldover artifacts. More specifically, depths (andcorresponding frequencies) outside of the displayed imaging range arenot aliased at least until they reach the outer bounds of the samplingimaging range. Additionally, fixed pattern noise arising near the DClimit 605 and the Nyquist limits 603 are advantageously reduced oreliminated. As utilized herein, fixed pattern noise is defined as noisethat does not exhibit a relationship with the frequencies representingthe sample. That is, noise that has an approximately fixed frequencythat is independent of the detected sample frequencies.

FIG. 7 is a block diagram of an example logic flow that may be performedby various portions of the system 500 of FIG. 5. In particular, thelogic flow depicts aspects of a method for optically imaging a sampleutilizing a frequency-domain OCT system. It is to be appreciated thatalthough the present disclosure describes the logic flow with referenceto the system 500 of FIG. 5, this is not intended to be limiting. Forexample, the logic flow may be implemented with an OCT system having adifferent configuration than the OCT system 500.

Referring now to FIG. 7, a flow diagram illustrating an exemplary method700 in accordance with the present disclosure is shown. In particular,the method 700 depicts a logic flow for optically imaging a sampleutilizing a frequency-domain optical coherence tomography (OCT) systemcomprising a wavelength-swept light source. The method 700 may begin atblock 710.

At block 710, illuminate a sample with a first portion of light; asample is illuminated with a first portion of light emitted by awavelength-swept light source. For example, the wavelength-swept lightsource 95 may illuminate the sample 136 with a first portion of lightemitted by the source 95.

Continuing from block 710 to block 720, illuminate a reference planewith a second portion of light; a reference plane is illuminated with asecond portion of light emitted by the wavelength-swept light source.For example, the wavelength-swept light source 95 may illuminate thereference mirror 82 with a second portion of light emitted by the source95.

It is to be appreciated, that the wavelength-swept light source 95 mayemit multiple modes and/or frequencies of light. The emitted light maybe divided into a first portion and a second portion by the coupler 72.The first portion and second portion of the emitted light is presentedto the sample arm 84 and the reference arm 80, respectively, toilluminate the sample 136 and the reference mirror 82.

Continuing from block 720 to block 730, shift a frequency of the firstportion of light or the second portion of light to define a samplingrange; a frequency of at least one of the first portion of light or thesecond portion of light is shifted to define a sampling range includinga set of frequencies. For example, the optical frequency shifter 311 mayshift the signal frequency band by a carrier frequency to define asampling range. More specifically, the optical frequency shifter 311 mayshift the signal frequency band by the carrier frequency 601 to definethe sampling range 650.

Continuing from block 730 to block 740, generate a displayed imagingrange from the sampling range; a displayed imaging range consistingessentially of only a subset of the set of frequencies within thesampling range is generated. For example, the signal processor 160 maygenerate the displayed imaging range by omitting frequencies within thesampling range where aliasing and/or foldover occurs.

Referring again to FIG. 6, as depicted, the sampling range 650 extendsfrom a first frequency to a second frequency and is substantiallycentered on the carrier frequency 601. Furthermore, the second frequencyis larger than the first frequency.

Additionally, the displayed imaging range 660 extends from a thirdfrequency larger than the first frequency to a fourth frequency smallerthan the second frequency and is also substantially centered on thecarrier frequency 601.

As noted above, in some examples, the frequency-domain OCT system 500may include band-pass filters. FIG. 8 is a plot 800 graphicallyillustrating an embodiment of the present disclosure incorporating aband-pass filter. In particular, the plot 800 illustrates incorporatingboth a high-pass filter 861 and a low-pass filter 863. As will beappreciated, in the plot 800, low frequencies represent shallow depthswhile high frequencies represent deeper depths.

Furthermore, it will be appreciated that in some OCT applications, ormore particularly with some OCT systems, reflections may be caused bythe imaging optics. These reflections may result in image degradation,such as, for example by producing peaks at lower frequencies or, moreproblematically, peaks folded in from the positive/negative depthambiguity and overlaid onto the displayed imaging range 660.

For example, FIG. 9 depicts a sample scan 910 performed by an exemplaryprobe 920. The probe 920 may be disposed on the sample arm of an OCTsystem (e.g., the sample arm 84). The probe 920 includes a fiber 921(e.g., single mode fiber (SMF)) optically coupled to a lens 922. Thefiber 921 and the lens 922 are disposed in a sheath 923. A drive shaft924 is coupled to the lens 922 to cause the lens 922 to rotate andexpose the sample 136 with light 930. The probe 920 may further includea balloon 940 used in the imaging process.

As can be seen from the sample scan 910, frequency peaks 911 a to 911 hare shown. These frequency peaks correspond to light reflected back fromthe sample 136 and also from the probe 920. In particular, the peaks 911a to 911 e may correspond to reflections from the sample 136 while peaks911 f to 911 h may correspond to reflections of the probe. Inparticular, the peak 911 f may correspond to a reflection from theballoon 940; the peak 911 g may correspond to a reflection from thesheath 923; and the peak 911 h may correspond to a reflection from thelens 922.

Returning to FIG. 8, the high-pass filter may be configured to filterout one or more of the peaks 911. In particular, the high-pass filter861 may be configured to filter out those peaks 911, such as, forexample, the peaks 911 f to 911 h. In some examples, the peaks 911 maycorrespond to shallower depths as discussed on conjunction with FIG. 10.

FIG. 10 is a plot 1000 graphically illustrating a shifting of thefrequency band by a carrier frequency (fs) 1001 in accordance with thepresent disclosure. In general, the frequency shift depicted isnegative. More specifically, the frequency shift is reversed withrespect to the frequency shift depicted in FIG. 6. Such a shift may beimplemented by, for example, reversing the direction of the wavelengthsweep, or using a negative frequency shift in the acousto-opticalfrequency shifter, or an alternative receiver design where thesample/reference arm phase shift is reversed during their interaction ata coupler or beam splitter. As a result, more negative frequenciescorrespond to deeper depths of imaging while more positive frequenciescorrespond to shallower imaging depths.

As depicted, the carrier frequency (fs) 1001 has been selected to extendthe Nyquist frequency (−f_(nyquist)) 1003 beyond a lower bound of an OCTquality envelope 1040. In particular, the sampling range 1050 includes aset of frequencies bound by the Nyquist frequency (−f_(nyquist)) 1003and the zero-frequency DC limit 1005. With some examples, reflectionsdue to imaging optics (e.g., as illustrated in FIG. 9) occur on thelow-pass filter 1063 side of the sampling range 1050. As a result, thelow-pass filter 1063 provides an image having significantly greaterattenuation and no ambiguity or foldover resulting from shallowreflections (e.g., as illustrated in FIG. 9). However, in some cases thenegative frequency-shift, or the negative carrier frequency (fs) 1001may increase the intensity of foldover artifacts resulting from thesample itself. More particularly, as the sample depths are on thehigh-pass filter 1061 or the zero-frequency DC limit 1005 side of thefrequency band, such artifacts may be increased.

FIG. 11 is a plot 1100 graphically illustrating an embodiment where thecarrier frequency is selected such that the sample range is shifted awayfrom the zero-frequency DC limit 1105. The embodiment shown in FIG. 11may be implemented to reduce or eliminate sample-originated foldoverartifacts arising from negative frequency shifts.

As depicted, the carrier frequency (fs) 1101 has been selected to extendthe Nyquist frequency (−f_(nyquist)) 1103 beyond a lower bound of an OCTquality envelope 1140 but moves the sampling range 1150 away from thezero-frequency DC limit 1105. In particular, the carrier frequency (fs)1101 is no longer approximately ½ nyquist, but instead may be selectedsuch that the sampling range 1150 extends from the first frequency(−f_(hp)) 1107 to the Nyquist frequency (−f_(nyquist)) 1103. Thehigh-pass filter 1161 may be implemented to filter frequencies betweenthe zero-frequency DC limit 1105 and the first frequency (−f_(hp)) 1107.As such, an increased resilience to foldover artifacts due to thepositive/negative frequency ambiguity of 2×fhp may be realized. In someexamples, the carrier frequency (fs) 1101 may be selected to besomewhere between the zero-frequency DC limit 1105 and the nyquistfrequency.

It is important to note, that although utilization of the carrierfrequency to provide additional shifting of the sampling range disclosedin the context of negative frequency shifts, this is not intended to belimiting. For example, a carrier frequency may be implemented to provideadditional frequency shift as described herein on conjunction with thesystems described with respect to FIG. 8 for example.

FIGS. 12A-12D illustrate example images acquired using an OCT system,and in particular, depict advantages enabled by various embodiments ofthe present disclosure.

Turning more specifically to FIG. 12A, an idealized image 1210 isillustrated. The idealized image may result from optimal sampleplacement and imaging utilizing the conventional configuration of FIG. 3and that described in the '497 patent referenced above. As will beappreciated, the idealized image 1210 of FIG. 12A lacks foldoverartifacts; however, such optimal sample placement is difficult or evenimpossible to achieve in practice.

Turning more specifically to FIG. 12B, a non-optimally placed image 1220is illustrated. The non-optimally placed image 1220 was obtainedutilizing the same configuration as the idealized image 1210. However,as depicted, the non-optimally placed image 1220 includes foldoverartifacts 1221 resulting from non-optimal sample placement. Suchfoldover artifacts are also depicted in FIG. 4 (e.g., refer to theartifacts 331). As will be appreciated, non-optimal sample placement mayresult from, for example, the sample arm not being positioned at asubstantially constant distance from the surface of the sample or tissuebeing imaged over a series of scans. As shown, the foldover artifactslead to spurious imaging data on at least a portion of the OCT scan.

Turning more specifically to FIG. 12C, a non-optimally placed image 1230is illustrated. The image 1230 was obtained utilizing variousembodiments of the present disclosure. In particular, the image 1230 wasobtained wherein the displayed imaging range was generated to beapproximately 50% of the sampling range. As can be seen, foldoverartifacts have been eliminated. In particular, selection of the carrierfrequency such that the sampling range is bound by the Nyquist frequencyand the zero-frequency DC limit and then generating the displayedimaging range as a subset of the frequencies from the sampling range hassubstantially reduced and/or eliminated aliasing and foldover artifacts.

As will be appreciated, however, the reduced displayed imaging range(e.g., as compared to the sampling range) may result in a portion of thescan of a non-optimally placed sample not being displayed. Inparticular, as can be seen from FIG. 12C, portions of the scan betweenpoints 1231 and 1232 are cut off. FIG. 12D illustrates the same scan ofa non-optimally placed sample with the display range being set to thefull imaging range. As shown, the depth range of the displayed scan isincreased compared to the scan of FIG. 12C, revealing the portion (e.g.,between points 1231 and 1232) of the scan previously lost due tonon-optimal sample placement.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

What is claimed is:
 1. A frequency-domain optical coherence tomography(OCT) system for optically imaging a sample, comprising: awavelength-swept light source configured to illuminate (i) the samplewith a first portion of light emitted by the wavelength-swept lightsource and (ii) a reference plane with a second portion of light emittedby the wavelength-swept light source; an optical frequency shifterconfigured to shift a frequency of at least one of the first portion oflight or the second portion of light to define a sampling range thatincludes a set of frequencies; and a signal processor configured togenerate a displayed imaging range based on the sampling range, thedisplayed imaging range consisting essentially of only a subset of theset of frequencies within the sampling range.
 2. The frequency-domainOCT system of claim 1, wherein the sampling range (i) extends from afirst frequency to a second frequency and (ii) is substantially centeredon a carrier frequency.
 3. The frequency-domain OCT system of claim 2,wherein the second frequency has a numerical value greater than anumerical value of the first frequency.
 4. The frequency-domain OCTsystem of claim 2, wherein the displayed imaging range (i) extends froma third frequency, having a numerical value greater than a numericalvalue of the first frequency, to a fourth frequency, having a numericalvalue less than a numerical value of the second frequency, and (ii) issubstantially centered on the carrier frequency.
 5. The frequency-domainOCT system of claim 4, wherein the fourth frequency has a numericalvalue that is greater than a numerical value of the third frequency. 6.The frequency-domain OCT system of claim 1, wherein the opticalfrequency shifter is configured to shift the frequency of the fiatportion of light based on a determined carrier frequency, wherein aNyquist frequency corresponding to the first portion of light isextended beyond either an upper or a lower bound of an OCT qualityenvelope corresponding to the first portion of light.
 7. Thefrequency-domain OCT system of claim 6, wherein the lower bound of theOCT quality envelope is greater than a zero-frequency DC limit of thefirst portion of light.
 8. The frequency-domain OCT system of claim 7,wherein the signal processor is configured to omit frequencies from theset of frequencies where aliasing occurs.
 9. The frequency-domain OCTsystem of claim 7, wherein the subset of frequencies in the displayedimaging range includes between 5 and 95 percent of the frequencies inthe set of frequencies in the sampling range.
 10. The frequency-domainOCT system of claim 7, wherein the subset of frequencies in thedisplayed imaging range includes between 25 and 75 percent of thefrequencies in the set of frequencies in the sampling range.
 11. Thefrequency-domain OCT system of claim 7, wherein the subset offrequencies in the displayed imaging range includes between 40 and 60percent of the frequencies in the set of frequencies in the samplingrange.
 12. The frequency-domain OCT system of claim 6, wherein theoptical frequency shifter is configured to shift the frequency of thesecond portion of light based on the determined carder frequency. 13.The frequency-domain OCT system of claim 6, wherein the carder frequencyis negative.
 14. The frequency-domain OCT system of claim 13, whereinthe optical frequency shifter is configured to provide a negativefrequency shift.
 15. A frequency-domain optical coherence tomography(OCT) system for optically imaging a sample, comprising: an opticalsource configured to emit an output light over a range of wavelengths;and an interferometer configured to detect an interference over asampling range of frequencies, wherein each of the range of frequenciesis associated with a different depth within the sample, wherein: theoptical source is further configured to illuminate (i) the sample with afirst portion of the output light and (ii) a reference plane with asecond portion of the output light, the interferometer is furtherconfigured to associate larger depths within the sample with morenegative frequencies within the sampling range, and the sampling rangeextends from a first frequency to a second frequency and issubstantially centered on a carrier frequency.
 16. The frequency-domainOCT system of claim 15, further comprising a signal processor configuredto generate an image of the sample based on a displayed imaging range,the displayed imaging range consisting essentially of a subset of thefrequencies of the sampling range.
 17. The frequency-domain OCT systemof claim 15, wherein the interferometer is further configured to shift afrequency of at least one of the first portion of light or the secondportion of light by a negative carrier frequency.
 18. Thefrequency-domain OCT system of claim 17, wherein the second portion oflight is shifted by the carrier frequency such that a correspondingsampling range extends from a first frequency to a Nyquist frequency,wherein the first frequency is different than a zero-frequency DC limit.19. The frequency-domain OCT system of claim 15, the signal processorfurther configured to filter reflections of the first portion of lightfrom the sample with at least one of a high-pass filter and a low-passfilter.
 20. A non-transitory computer-accessible medium providingthereon computer-executable instructions which for optically imaging asample using a frequency-domain optical coherence tomography (OCT)system, wherein, when the computer-executable instructions are executedby a computer hardware arrangement, the computer-executable instructionsconfigure the computer hardware arrangement to performs procedurecomprising: controlling a wavelength-swept light source generate light,wherein (i) the sample is illuminated with a first portion of lightemitted by the wavelength-swept light source and (ii) a reference planeis illuminated with a second portion of light emitted by thewavelength-swept light source, and a frequency of at least one of thefirst portion of light or the second portion of light is shifted usingan optical frequency shifter to define a sampling range that includes aset of frequencies; and controlling a signal processor to generate adisplayed imaging range based on the sampling range, the displayedimaging range consisting essentially of only a subset of the set offrequencies within the sampling range.
 21. A non-transitorycomputer-accessible medium providing thereon computer-executableinstructions which for optically imaging a sample using afrequency-domain optical coherence tomography (OCT) system, wherein,when the computer-executable instructions are executed by a computerhardware arrangement, the computer-executable instructions configure thecomputer hardware arrangement to performs procedure comprising:controlling an optical source to emit an output light over a range ofwavelengths; and controlling an interferometer to detect an interferenceover a sampling range of frequencies, wherein each of the range offrequencies is associated with a different depth within the sample,wherein: the optical source illuminates (i) the sample with a firstportion of the output light and (ii) a reference plane with a secondportion of the output light, the interferometer is controlled toassociate larger depths within the sample with more negative frequencieswithin the sampling range, and the sampling range extends from a firstfrequency to a second frequency and is substantially centered on aearner frequency.